The cardiac cycle can be described as the activation of certain specialized heart conduction cells in a predictable sequence, which leads to a coordinated and sequential contraction of the arterial and ventricular muscle fibers. The electrical signal associated with the muscle action is transmitted through various tissues and ultimately reaches the surface of the body, where it can be measured. Such a measurement is called ECG which stands for electrocardiogram.
Electrical equipment for such measurements is used for monitoring and/or recording ECG data and may be stationary or portable.
The stationary ECG equipments are electrical monitoring and recording devices which are connected to a patient by wires. In current use, such monitors utilize surface electrodes located on the body of the patient and connected by wires to an electrocardiograph machine, which allows the detected heart signals to be displayed on a paper strip or a monitor. However, the use of such wiring limits the mobility of the patient and requires the patient to remain in bed throughout the monitoring.
The portable ECG equipments can be divided into recorders and transmitters. In both cases wires from multiple electrodes applied to the body of a patient are connected to a recorder or transmitter unit, hung around the patient's neck. These units are often burdensome. The recorder unit is a self-contained unit such that the patient may move around. The transmitter unit further contains some sort of radio equipment, which makes it possible for the patient to move around and still be monitored by a stationary unit receiving the measuring data signals from the transmitter unit.
A common problem with electrodes and wires is the risk of detachment from the patient, as the wires and electrodes may be exposed to high tensile forces, the total required length of wires often being in the range of one meter. Moreover, the measurements and displayed curves may be influenced by physical pressure or strains on the electrodes.
Thus, the problem with wires or electrodes that are pulled off remains with all the described equipments.
Prior art systems of the above described types are disclosed in inter alia U.S. Pat. Nos. 4,243,044; 5,427,111; 6,026,321; 6,416,471; 6,453,186; 6,494,829; 6,526,310; 6,551,252; 6,567,680; 6,579,242; 6,589,170 and 6,611,705.
Methods of acquiring ECG have been proposed to exclude all wires and integrate the ECG recording and radio transmitter in every sensing unit, e.g. disclosed in U.S. Pat. Nos. 3,943,918; 4,981,874; 5,168,874; 5,307,818; 5,862,803; 5,957,854; 6,289,238; 6,132,371; 6,441,747; 6,496,705 and 6,577,893.
In U.S. Pat. Nos. 4,850,370; 5,058,598 and 6,901,285, the basic idea is based on first transforming a reduced number of the ECG measurements to one equivalent X, Y, Z source and then deriving the standard leads by using the equivalent voltage source and the “known” impedance of the body. The major problem with this approach is that the impedance variation between people is large. Therefore, the ECG estimate will sometimes be totally wrong, if the imped-ance is not measured. Thus, the ECG signal measured with these proposed solutions is incorrect and can not be used in for diagnosis. These systems have therefore never come to clinical use.
U.S. Patent Appl. Publ. No. 2002/0045836 A1 discloses a surveillance system for wireless transfer of signals from a number of electrodes positioned on a subject and each having its own contacts to a base station being capable of controlling the electrodes in several respects. More precisely, four electrodes positioned in the corners of an elongate rectangle the long sides of which are parallel to the direction of the standard lead and the short sides of which are positioned substantially in where the standard leads are detected and orthogonal to the long sides. The electrode contacts of each short side are elements of a separate electrode.
In U.S. Patent Appl. Publ. No. 2002/0045838 A1 the body is assumed to act solely resistively and homogenously which is a major simplification leading to the use of a predetermined scalar factor that will give a false and clinically useless standard lead.